Field of the Invention
The present invention relates to a measurement apparatus, a lithography apparatus, and a method of manufacturing an article.
Description of the Related Art
When manufacturing a device such as a semiconductor element, liquid crystal display element, or thin-film magnetic head using a photolithography technique, an exposure apparatus that projects the pattern of a reticle to a substrate such as a wafer by a projection optical system and transfers the pattern is used.
Along with downsizing and growing in demand of an electronic device, an exposure apparatus needs to achieve miniaturization and productivity improvement of semiconductor elements represented by a memory and MPU. The exposure apparatus is, therefore, requested to improve basic performance such as a resolution, overlay accuracy, and throughput. Since the resolution of the exposure apparatus is inversely proportional to the numerical aperture (NA) of a projection optical system and is proportional to the wavelength of light (exposure light) used for exposure, the numerical aperture of the projection optical system has been increased and the wavelength of the exposure light has been shortened. Along with miniaturization of semiconductor elements, it is necessary to improve the overlay accuracy, and thus it is also necessary to improve the accuracy of alignment of the relative positions of a reticle and substrate.
As a technique for alignment of a substrate, there is known a technique of performing two kinds of alignment, that is, pre-alignment and fine alignment using an alignment mark provided on the substrate. In pre-alignment, the position shift amount of the substrate fed from a substrate conveyance system to a substrate stage is detected, and the substrate is roughly aligned (positioned) so as to start fine alignment. In fine alignment, the position of the substrate held by the substrate stage is accurately measured, and the substrate is finely aligned (positioned) so that the alignment error of the substrate falls within an allowance.
In pre-alignment, as described above, it is necessary to detect the position shift amount of the substrate fed from the substrate conveyance system to the substrate stage. An alignment optical system for detecting an alignment mark has a wide detection area (field) with respect to a mark size. Pattern matching (template matching) processing is often used to obtain the position (X and Y coordinates) of the alignment mark from the wide detection area.
In general, pattern matching processing is roughly classified into the following two types. One is a method of binarizing an image (grayscale image), performing matching with a template prepared in advance, and sets a position having the highest correlation as the position of an alignment mark. The other is a method of obtaining the position of an alignment mark by performing correlation calculation for a grayscale image with a template including grayscale information. In such pattern matching processing, it is difficult to detect an alignment mark with respect to a low-contrast image, a noise image, or an image including a defect caused when processing the substrate. To solve this problem, Japanese Patent Laid-Open No. 2000-260699 proposes a technique of allowing stable detection for an image in which it is difficult to detect an alignment mark. The technique disclosed in Japanese Patent Laid-Open No. 2000-260699 has as its feature to simultaneously extract the edge of a mark and the direction of the edge, and perform pattern matching by paying attention to the edge for each edge direction.
There are provided a movement measurement method and image processing method as alignment methods. In the movement measurement method, the position of an alignment mark provided on a substrate is obtained by illuminating the alignment mark with light (laser), and in parallel measuring a change in intensity of light reflected by the alignment mark and the position of a substrate stage while moving the substrate stage. In the image processing method, the position of an alignment mark provided on a substrate is obtained by illuminating the alignment mark with white light while a substrate stage stays still, detecting light reflected by the alignment mark using charge-accumulation type photoelectric conversion elements, and performing image processing.
As an alignment optical system used in such alignment methods, there are provided a TTL (Through-The-Lens) optical system, TTR (Through-The-Reticle) optical system, and off-axis optical system. The TTL optical system detects an alignment mark provided on a substrate via a projection optical system. The TTR optical system simultaneously detects an alignment mark provided on a reticle and an alignment mark provided on a substrate via a projection optical system. The off-axis optical system is a dedicated optical system having an optical axis at a predetermined distance from the optical axis of a projection optical system without intervention of the projection optical system, and detects an alignment mark provided on a substrate by emitting white light from a dedicated light source.
For example, in pre-alignment described above, the position of an alignment mark is obtained by performing pattern matching processing for the image information of all the pixels (a wide detection area with respect to a mark size) of charge-accumulation type photoelectric conversion elements. Therefore, pattern matching processing is performed for pixel information corresponding to a wide detection area including a region where no alignment mark exists. A time corresponding to the detection area is taken to obtain the position of the alignment mark.